FIELD OF THE INVENTION
The subject method of fabricating laminated composite materials and materials fabricated thereby generally relate to structural materials having high resistance to material failure under tension. More specifically, the method of fabricating laminated composite materials combines a ceramic composition and a metallic composition into a laminate exhibiting the high strengths generally characteristic of ceramics and the toughness generally characteristic of metals.
The use of ceramics as structural materials is well-known. It is well recognized that ceramics, when free of material faults such as scratches and cracks, generally exhibit great strength. Typically, a ceramic such as silicate glass, in its pure, fault-free form exhibits a yield strength of nearly 3600 MPa/m.sup.2. Considering that most metals exhibit yield strengths no greater than 2000 MPa/m.sup.2, the use of ceramics particularly for high stress structural applications holds much promise.
The yield strength of ceramics, however, is heavily dependent on the absence of faults in the material. Moreover, ceramics generally are quite vulnerable to the formation upon impact of cracks and even more vulnerable to the propagation therethrough of such cracks, once they are formed. That is, while ceramics do exhibit great yield strength, they also exhibit extremely low fracture toughness and toughness. Compared to the 100-350 MPa/m.sup.2 and 100-1000 kJ/m.sup.2 fracture toughness and toughness values seen typically in most metals, ceramics such as silicate glass exhibit fracture toughness and toughness values of only 0.7 MPa/m.sup.2 and 0.01 kJ/m.sup.2. Thus, the great yield strengths ideally realizable with ceramics are, in practice, extremely difficult to actually realize, given their inherent susceptibility to fracture formation.
The use of metals in structural materials is, obviously, also well-known. While metals generally do exhibit much greater fracture toughness and toughness compared to ceramics, as mentioned above; they simply do not exhibit a great enough yield strength to be useful in many high stress structural applications. Most metals which exhibit strengths in the upper regions of the yield ranges, are typically specialized metal alloys which, in most cases, may be obtained only at high cost.
Another factor limiting the use of metals in extremely high stress applications, aside from their limited yield strengths, is their weight density. In many applications, the high weight density of metals is a tolerable, though undesirable, trade-off factor. In many high performance applications, however, the high density of such metals is not tolerable, even if the strength offered by the metal were acceptable. Thus, structural materials which exhibit at least the overall toughness and yield strengths of metals, yet do not exhibit the high weight densities of metals would be a highly desirable feature in most structural applications and is found to be necessary in many higher performance applications.
Efforts have been made in the past to augment the desirable properties realized in a structural material by combining layers of similar or dissimilar compositions into a structural laminate. For instance, ancient metallurgical processing techniques included the folding of metallic sheets into multi-layered structures. This technique tended to yield materials having increased tensile strength (compared to the tensile strength of any individual folded sheet), but was plagued in its simplicity by the inability to effect true, consistent layering and to maintain across each layer uniformity of thickness and material composition. This made precise control of the resulting material's final bulk properties virtually impossible.
Another metallurgical processing technique employed in the past was the simultaneous rolling of dissimilar metallic sheets stacked together. This technique has traditionally been confronted with a number of significant problems arising from the simple mechanical rolling of, invariably, complex composites. Among these problems which, to this day, have not been sufficiently overcome in employing this technique, are differential thermal expansions of the constituent metals, insufficient interlayer bonding, non-uniformity in the plastic flow of the layers, and cross diffusion among the layers. A significant by-product of these problems is a practically unavoidable non-uniformity in the resulting laminate material's thickness or width which necessarily diminishes the quality, salability, stress repeatability, and consistency of the bulk structural properties realized in the laminate material. What is more, the ever-present weight density problem of metals is not addressed by this technique.
A concept that has emerged more recently is that of combining layers of metal and ceramic to form composite laminated structures commonly referred to as laminated "Cermets." The potential to concurrently realize in such structures a material having desirable structural properties of both metals and ceramics, yet having a tremendously high strength-to-weight ratio, has spurred extensive study and developmental effort. Numerous techniques for forming laminated Cermets have developed as a result, including: powder sintering, electrolytic growth, vapor deposition, plasma spraying, and mechanical stacking and pressing. These techniques have in various respects proven inherently ineffective and inefficient, such that the mass production of laminated Cermets at any useful level has yet to be realized. These techniques have also failed to yield any Cermet panels of sufficiently great surface area to prove useful in commercial applications. Hence, these existing techniques find severely limited, if any, utility where useful forms of Cermet materials are to be generated within realistic cost constraints.